It is substance placed strategically so that the Pest may approach it and eat it considering a food source and end up being poisoned. Baits are made to be palatable to the Pest and formulated to last longer than normal fresh foods.
Most baits are NOT instantly lethal to the target Pests. The advantage is that the bait may be taken to the Pest population that may not leave the nest and that the Pest would like die off in the nest and not in our living spaces.
Since most baits are formulated such that they do not cause any immediate discomfort to the Pest; the Pest is likely to have multiple feedings and encourage other members to feed as well.
Examples: There are baits in GEL or PASTE formulations to target ants and cockroaches whereas to target mice, the bait formulation is Block.